Interruption of the aortic arch is a rare anomaly affecting 1% of children with congenital heart disease. The systemic circulation is ductal dependent and is determined principally by the ratio of the resistances in the systemic and the pulmonary vascular bed. Any increase in the pulmonary vascular resistance may increase the dead space ventilation due to acute pulmonary hypoperfusion. We report a case where sudden decreases in the end-tidal carbon-dioxide due to pulmonary hypoperfusion mimicked accidental endotracheal tube extubation in an infant undergoing repair of interrupted aortic arch.

Interruption of the aortic arch (IAA) is a rare anomaly affecting 1% of children with congenital heart disease (CHD). [1] Systemic to pulmonary blood flow in these infants is determined principally by the ratio of the resistance in the systemic and pulmonary vascular bed. There are no previous reports that describe an abrupt decrease in end-tidal carbon-dioxide (E T CO 2 ) due to pulmonary hypoperfusion mimicking accidental endotracheal tube extubation in infants undergoing surgical repair of IAA.

Case Report

A 6-day-old male child weighing 2.5 kg underwent repair of IAA at our institution. He was born by normal vaginal delivery at 34 weeks of gestation and presented with tachypnea, tachycardia, and feeding difficulty shortly after birth. Transthoracic echocardiography revealed situs solitus, levocardia, left arch, type B IAA (1 cm interruption between the left subclavian artery and the left carotid artery), large (8-10 mm) sub-pulmonic doubly committed ventricular septal defect (VSD) shunting left to right, restrictive (3 mm) atrial septal defect, and a 3.6 mm patent ductus arteriosus (PDA) originating from the main pulmonary artery (PA) trunk and continuing as the descending thoracic aorta (DTA), with juxta-ductal origin of the left subclavian artery from the DTA. The diameter of the ascending aorta was 7 mm while that of the PA measured 15 mm. A bi-directional shunt was present across the PDA with a gradient of 22 mmHg.

The pulmonary venous return was normal with normal concordance of the atrioventricular and ventriculoarterial valves. There was severe pulmonary artery hypertension (PAH), but no left ventricular (LV) outflow tract obstruction. The infant also had syndromic facies suggestive of DiGeorge syndrome with micrognathia, bulbous nose, and low set ears. The child was referred to our institution in cardiogenic shock and severe metabolic acidosis with deranged hepatic and renal parameters probably due to impending closure of the PDA [Table 1]. His trachea was intubated with a 3 mm uncuffed portex endotracheal tube and mechanical ventilation was instituted with fractional inspired oxygen concentration (FiO 2 ) of 30%. Preoperative stabilization was attempted with prostaglandin E 1 (PGE 1 , 0.05 μg/kg/min) and dopamine (8 μg/kg/min), but as there was minimal improvement in renal and hepatic parameters, the infant was taken up for urgent surgery.

Table 1: Biochemical, respiratory and metabolic parameters at various time points

In the operation theatre, a leak was noticed in the delivered tidal ventilation of about 10-12 ml (inspiratory tidal volume 25 ml, expiratory tidal volume 12-15 ml) in the Datex Ohmeda Ventilator panel (Datex Ohmeda Aestiva S/5, Madison, USA). This was associated with an audible leak; the existing tube was changed to 3.5 mm uncuffed endotracheal tube when difficulty to visualize the vocal cord was encountered. The passage of the endotracheal tube into the trachea could not be visualized; however, intratracheal position was confirmed with the E T CO 2 trace. The tube was fixed at 8.5 cm mark at the lips. Bilateral auscultation confirmed equal air entry in both lungs with E T CO 2 value of 52 mmHg. The peak inspiratory airway pressure was 15 cm H 2 O. An arterial blood gas sample at this point showed a pH of 7.269, PO 2 of 72 mm Hg on FiO 2 of 30%, partial pressure of arterial carbon dioxide (PaCO 2 ) of 60 mmHg, oxygen saturation (SaO 2 ) of 91.5%, base deficit of −6.7 mEq/l and serum ionized calcium of 0.69 mmol/l. The saturation of oxygen in the lower limb measured 88%.

Just before the infant was to be handed over to the surgeons, without any apparent stimulation, the E T CO 2 values declined acutely to ~ 5 mmHg, which was followed by bradycardia (heart rate dropped from 130 beats/min to 90 beats/min) and hypotension (femoral artery systolic blood pressure dropped from 62 mmHg to ~ 30 mmHg). The child was immediately hyperventilated manually. Although the chest was noted to be expanding during ventilation, the feel in the bag was 'tight' and auscultation for bilateral air entry proved inconclusive because of vague conducted sounds.

At this point, accidental extubation of the endotracheal tube was suspected. Just as laryngoscopy was about to be performed to confirm the tube position, there was a sudden return of the E T CO 2 trace followed by stabilization of the hemodynamics. The entire episode probably lasted 15 seconds. A decision was taken to proceed with laryngoscopy which failed to show clear passage of the endotracheal tube through the vocal cords despite adequate cricoid pressure. A sterile suction catheter was then passed gently through the endotracheal tube without any resistance. Suctioning revealed moderate amount of thin mucoid secretions. Shortly afterward, just before skin incision, a second episode of sudden drop in E T CO 2 followed by bradycardia and hypotension occurred which resolved spontaneously before any intervention.

Cardiopulmonary bypass (CPB) using a roller pump (Sarns 8000, Terumo, US) and membrane oxygenator (Capiox Baby RX, Terumo, Japan) was established by biarterial-venous cannulation; the ascending aorta and the PDA (through the PA) were cannulated separately while a venous cannula was inserted in the right atrium. The patient was cooled to a nasopharyngeal temperature of 18°C over 20 minutes. A cardioplegia cannula was inserted into the ascending aorta and cardioplegia administered after the aorta was cross-clamped. 1/3 rd of the estimated blood volume of the patient was then drained into the venous reservoir and total circulatory arrest (TCA) established. The PDA was divided on TCA and an end-to-side anastomosis of the DTA with the undersurface of the arch performed. The aortic cannula was reinserted into the ascending aorta after completion of the anastomosis and flows restarted slowly and the arch vessels unsnugged. The patient was rewarmed to a nasopharyngeal temperature of 24°C, and the VSD repaired using a Gore-Tex patch through a transverse atriotomy in the PA.

The infant separated from CPB with inotropic support (adrenaline 0.05 μg/kg/min, noradrenaline 0.03 μg/kg/min and milrinone 0.5 μg/kg/min) and sub-systemic PA pressures (systemic arterial blood pressures ~ 50/30-60/30 mm Hg and PA pressures ~ 35/20-40/20 mm Hg). But there was diffuse oozing from all the tissues including needle hole bleeding from the suture lines on the aorta. In addition, the post bypass serum ionized calcium persistently remained low. Although this was managed with blood and products transfusion as well as calcium infusion and increasing doses of inotropes, the infant continued to ooze in the postoperative intensive care and developed profound acidosis [Table 1] and severe myocardial dysfunction and died 2 hours later.

Discussion

In normal individuals, E T CO 2 not only reflects adequacy of ventilation, it is also a measure of the pulmonary perfusion. However, in patients with CHD, there exist large gradients between the PaCO 2 and the end-tidal exhaled alveolar gas carbon-dioxide (P ET CO 2 ). [2],[3] In patients with cyanotic CHD, the difference is related to both increases in dead space ventilation and intracardiac venous admixture, whereas in patients with acyanotic CHD, the PAH resulting from the increased pulmonary blood flow may cause a greater ventilation perfusion mismatch resulting in increased PaCO 2 -P ET CO 2 gradients. [4] This difference may be further exaggerated in anesthetized and paralyzed patients with PAH due to atelectasis and increase in dead space ventilation. Despite these caveats, under conditions of constant ventilation, E T CO 2 remains a reliable indicator of pulmonary blood flow. [5],[6]

Our hypothesis is that the sudden decline in the E T CO 2 trace in our patient was probably due to an abrupt increase in the PA pressures leading to a PAH crisis and impending/transient cardiac arrest. In patients with excessive pulmonary blood flow, the pulmonary vascular bed is highly reactive which makes them susceptible to episodes of PAH crises. A crisis can be precipitated by any noxious stimuli such as acidosis, hypercarbia, hypoxia, pain or by simply suctioning the trachea. Occasionally, no obvious precipitating cause can be observed. [7] A PAH crisis is characterized by a sudden rise in PA pressures which may become supra-systemic. [7] The conventional triad of a PAH crisis consists of bradycardia, hypotension, and/or hypoxemia; fall in E T CO 2 occurs last and is secondary to cardiac arrest. [7]

In IAA, right ventricle (RV) pressures are systemic. [8] Since the RV ejects into both pulmonary and the post-ductal systemic circulations, a PAH crisis would first be manifested as a drop in E T CO 2 trace due to pulmonary hypoperfusion followed by bradycardia and hypotension due to subsequent fall in systemic output. During a crisis, the bi-directional shunt across the PDA as well as the intracardiac left-to-right shunt across the VSD reverse and become right to left, thus maintaining pre- and postinterrupted systemic output for a brief transient period before hypoxemia supervenes. Since the saturation and pressures in the PA is equal to that of ascending aorta, a transient PA crisis may not result in difference in saturation and pressures between the pre- and post-interrupted systemic circulations [Figure 1]a and [Table 2]. [8] The second episode of near total loss of E T CO 2 can also be explained by another episode of transient cardiac arrest due to a second PAH crisis. In fact, in patients with elevated PA pressures, clusters of PAH crises are observed frequently. [7]

Figure 1: Schematic diagram showing the distribution of blood flow between the systemic and pulmonary circulation in infants with type B interrupted aortic arch in two situations. In (a), a PA crisis shunts blood from right to left across the PDA (straight arrow) and the VSD (curved arrow) at the expense of pulmonary blood flow (crossed lines). Thus no/minimal saturation and pressure differential (dashed arrows) exists in pre- and postinterrupted circulations. In (b), the reverse situation occurs with acute closure of PDA (crossed line); the ductal dependant systemic output drops and a pressure differential is discernable between the ductal (post-interrupted) and nonductal (pre-interrupted) circulations (dashed arrows). The shunting across the VSD would depend on the relative compliances of the left and right ventricle and may become bidirectional (bidirectional arrow). RV - right ventricle; RA - right atrium; MPA - main pulmonary artery; LPA - left pulmonary artery; RPA - right pulmonary artery; PDA - patent ductus arteriosus; AO - ascending aorta; DTA - descending thoracic aorta

Conversely, if the PDA begins to close first, the PDA-dependent systemic output drops leading to bradycardia and hypotension. Since the cardiac output to the non-ductus circulation is maintained transiently due to the increased pulmonary venous return to the LV, closure of the PDA would be manifested by weakened pulses and decreased saturation in the ductus-dependent systemic circulation as compared to the nonductus systemic circulation [Figure 1]b and [Table 2].

The main stay of management in PAH crisis is manual hyperventilation to reduce the pulmonary vascular resistance (PVR) [Table 2]. The primary aim should be to lower the PaCO 2 so that PVR comes down and improves the pulmonary blood flow. Once the PVR is reduced and the child stabilized, a set tidal volume can be delivered by the volume control mode since with pressure preset modes, the delivered tidal volume may vary depending on lung compliance. Additional measures to decrease PVR should include correction of metabolic acidosis, use of pulmonary vasodilators, adequate analgesia and avoidance of precipitating factors such as suctioning during a crisis with circulatory support being reserved for unresolved life threatening attacks. In patients with impending acute closure of the PDA however, hyperventilation may be counter-productive as it would shunt more blood to the lungs as well as hasten closure of the PDA; in such cases, prostaglandins, inotropes and circulatory support constitute the mainstay of management [Table 2].

The fact that hyperventilation led to a restoration of the E T CO 2 trace is further evidence that the cause of acute reduction in the pulmonary blood flow in our patient was a PAH crisis. Although hyperventilation may worsen the hypocalcemia in patients with DiGeorge syndrome, preference should be given to hyperventilating these patients in the event of a PAH crisis, with correction of the ionized calcium levels by infusion if necessary.

The differential diagnosis in this scenario would include accidental endotracheal extubation, kinking or blockade of the endotracheal tube, mechanical disconnection in the breathing circuit, acute severe bronchospasm, cardiac arrest and pulmonary thromboembolism. [6] Although accidental endotracheal extubation is usually diagnosed by absence of breath sounds over the chest and a sudden loss in the E T CO 2 trace, it may be difficult to establish it in a patient with IAA.Provided the situation permits, a fiberoptic bronchoscopy would be required for a definite confirmation of the tube position. If in the event a pediatric fiberscope cannot be passed through the endotracheal tube, it can be inserted in the oral cavity for visualizing the tube entering the cords. In our case, we did not have the pediatric fiberscope at our disposal.

Blockade or kinking of the endotracheal tube and mechanical disconnections can alter the E T CO 2 trace rapidly. While mechanical circuit disconnection is easily recognized, at least 50% occlusion of the endotracheal tube is required for the E T CO 2 trace to change. [6] Total occlusion of the endotracheal tube leads to a loss of the E T CO 2 trace along with rise in peak airway pressures, whereas subtotal occlusion or kinking of the tube will lead to a diminished E T CO 2 trace with prolongation or slanting of the expiratory upstroke (phase II) of the capnograph along with a rise in the peak airway pressures. [6] A similar picture will also be seen in cases of acute asthma and/or bronchospasm. Another rare situation that can mimic a sudden drop in E T CO 2 could be massive pulmonary thromboembolism, but along with acute increases in PA pressures, this would lead to a sudden increase in the end-tidal nitrogen fraction (i.e. the percentage of balance end-tidal gas) on the gas monitor. Finally, since CO 2 elimination from the alveolar gas is a function of the cardiac output, a cardiac arrest will also result in a sudden loss of the E T CO 2 trace.

A PAH crisis may result in elevation of the airway pressures secondary to the transmural transmission of the PA pressures to the alveoli; in this case, although the feel of the bag during manual hyperventilation during the first episode was tight, we cannot comment definitely on the airway pressures and the second episode was very transient and resolved spontaneously. In the inability to ventilate these patients either manually or mechanically during a crisis, a suction catheter may be passed gently inside the endotracheal tube to rule out any blockade. However, caution should be exercised in passing the catheter through the tube as it may stimulate the airway leading to exaggeration of the PAH crisis.

Conclusion

In infants with IAA, since the RV ejects into both the pulmonary and the post-ductal systemic circulations, abrupt increases in PA pressures may decrease the E T CO 2 trace first due to pulmonary hypoperfusion before bradycardia and systemic hypotension supervene. This scenario may mimic accidental endotracheal tube extubation. Since most of these children have associated DiGeorge syndrome with difficult airway anatomy, it is important to optimize the PVR quickly before proceeding with the standard dictum for doubtful endotracheal tube position, i.e. "when in doubt, pull the tube out."

Acknowledgement

We would like to acknowledge Ms. Vasanthy S., senior artist, SCTIMST for the schematic diagram.